In a mobile communications environment, reception signals can undergo amplitude variations and phase variations due to Rayleigh fading attendant upon changes in the relative positions of the base station and the mobile station. In phase modulation systems, typically, data is transmitted by the carrier wave phase of symbols before and after operational coding, and, on the receiving end, information data is recognized and read using differential detection. However, because differential detection involves operational coding of the transmission data as described above, a 1-bit error in the wireless interval amounts to a 2-bit error in the information data, so in binary phase-shift keying systems (BPSK modulation), which involve synchronous detection, the signal power to noise power ratio (SNR) will be degraded for the same bit error rate (by, for example, approximately 3 dB).
Additionally, although absolute synchronous detection for measuring a reception signal phase at an absolute phase of each individual symbol makes for highly efficient reception, it is difficult to determine a reception signal absolute phase in a Rayleigh fading environment.
Thus, for example, a method of estimating and correcting fading distortion using pilot symbols of known phase inserted at regular intervals between data symbols has been proposed (Sampei, “16 QAM Fading Distortion Correction for Terrestrial Mobile Communications”, Denshi Joho Tsushin Gakkaishi, Vol. J72-B-II No. 1, pp. 7-15, January 1989). In this method, one pilot symbol of known transmission phase is inserted every several data symbols in a communications channel and the channel estimated using the received phase of the pilot symbol. That is, amplitude and phase of a reception signal for each transmission user at pilot symbols at the beginning and end of such data symbol interval are measured, and a variation in the channel for the data symbol interval is estimated by interpolation between the measured values.
Additionally, in mobile communications systems according to the IS-95 (CDMA transmission system) currently in service, the downlink involves using a pilot channel common to all users, with encoded multiplex signal transmission accomplished by using a code orthogonal to a spreading code for each user. At the receiving end, the pilot channel and the data channel are separated by despreading, a channel variation is estimated using the pilot channel, and the data symbols are demodulated using the estimation results.
However, in a multi-carrier CDMA transmission system, a plurality of sub-carriers are used to perform data symbol transmission, with different channel variations for each sub-carrier. Additionally, the transmission signal is spread in the sub-carrier direction, so at the receiving end it is necessary to make an estimation for each sub-carrier at the chip level prior to despreading. As a result, it is not possible to employ directly the channel variation estimation method employed in transmission systems that use direct spreading (DSCDMA) such as IS-95 described above in a multi-carrier CDMA transmission system.
Additionally, in a direct-sequence spreading system (DS-CDMA) such as IS-95, in which it is assumed that transmission takes place with each user (each mobile station) using the same channel, it is possible to estimate the channel by multiplexing the pilot channel common to all users. However, in cases, for example, in which communications are conducted by using a user-specific beam pattern for each user (that is, each mobile station) using an adaptive array antenna, the common channel and the channel for individual users differ, so channel estimates for the common pilot channel cannot be used for the channels for individual users.